How a Moped CDI Works

Written by chris gilliland | 13/05/2017

Mopeds are usually powered by an electronic ignition system called a CDI, or Capacitor Discharge Ignition. The CDI box is composed of an electrical transformer, a rectifier, a capacitor and a series of circuits. Capable of producing a high-voltage spark to insure a strong combustion, the CDI requires an adequate power source. Power, in the form of an electrical current, is produced by the moped's battery, which is fed to various components within the moped, including the stator, or alternator. The stator is composed of a ring of tightly wound metal wires that generate an electrical field from the battery's current. A set of magnets are mounted on the engine's flywheel, which is attached to the engine's crankshaft and housed within the stator. As the flywheel spins, it produces an alternating current that is fed back to the battery to complete the moped's charging system.

Capacitor Charging

As the stator produces power in conjunction with the flywheel, the current is transmitted to the Capacitor Discharge Ignition (CDI) box. The current from the stator is amplified by a transformer within the CDI to around 400 to 600 volts before flowing through a charging circuit and into a capacitor, an electronic component used to temporarily store energy.

Ignition and Timing

The ignition timing is a preset interval used to fire the spark plug at the right time to maximise combustion. On most mopeds, timing is set a few degrees before the engine's piston reaches the top of its travel, referred to as Top Dead Center, and is often triggered by a magnetic or mechanical trigger circuit. This circuit stops the charging cycle of the capacitor, discharging the energy stored within the capacitor through the ignition coil wire. Ultimately, the high-voltage current is transmitted into the spark plug to create a spark at its tip to create combustion.

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